Effects of urban to regional emissions on the air pollution health burden in Europe
Authors: Daven K Henze
Journal: The Lancet Public Health (2023)

Ambient air pollution is one of the largest health risk factors facing populations globally;1Cohen AJ Brauer M Burnett R et al.Estimates and 25-year trends of the global burden of disease attributable to ambient air pollution: an analysis of data from the Global Burden of Diseases Study 2015.Lancet. 2017; 389: 1907-1918Summary Full Text Full Text PDF PubMed Scopus (3543) Google Scholar however, who is responsible for pollution exposure across various populations is unclear. Is attainment of clean air the provenance of municipal governments, or does it require coordinated action with neighbouring upwind cities, states or provinces, and countries, or even intercontinental agreements? Many studies have sought to address this question, from investigations of hemispheric atmospheric transport affecting continental mean pollutant concentrations2Dentener F Keating T Akimoto H Hemispheric transport of air pollution 2010. Part A: ozone and particulate matter. UN, New York, NY, and Geneva2010Google Scholar to quantifying the role of municipal emissions in air quality in individual cities.3Tessum MW Anenberg SC Chafe ZA et al.Sources of ambient PM2·5 exposure in 96 global cities.Atmos Environ. 2022; 286119234Crossref Scopus (4) Google Scholar, 4Nawaz MO Henze DK Anenberg SC et al.Sources of air pollution-related health impacts and benefits of radially applied transportation policies in 14 US cities.Front Sustain Cities. 2023; 51102493Crossref Scopus (0) Google Scholar Not surprisingly, the answer to who is responsible is often that it depends—which pollutant, which region, and even which timescale (ie, episodic vs long term). A challenge for air quality researchers is thus studying myriad situations and providing tools to answer customised source attribution questions posed by stakeholders at various scales, at locations worldwide, with the benefit that these efforts can help to synergise policy objectives of climate, health, and air quality.5Kleiman G Anenberg SC Chafe ZA et al.Enhanced integration of health, climate and air quality management planning at the urban scale.Front Sustain Cities. 2022; 4934672Crossref Scopus (1) Google Scholar The work of Sasha Khomenko and colleagues, published in The Lancet Public Health, brings a new perspective to this problem.6Khomenko S Pisoni E Thunis P et al.Spatial and sector-specific contributions of emissions to ambient air pollution and mortality in European cities: a health impact assessment.Lancet Public Health. 2023; 8: e546-e558Google Scholar The authors quantify the contribution of emissions from different sectors to the air pollution health burden (ie, premature mortality associated with long-term exposure to PM2·5 and NO2) in 857 European cities. For each city, this source attribution is broken down by contributions from the city itself, its country, and the rest of Europe. Earlier work with the Screening for High Emission Reduction Potentials for Air Quality tool7Thunis P Degraeuwe B Pisoni E et al.PM2·5 source allocation in European cities: a SHERPA modelling study.Atmos Environ. 2018; 187: 93-106Crossref Scopus (56) Google Scholar focused on only 150 urban areas, used older inputs (ie, emissions), and did not include NO2. Other global-scale modelling studies have had less representation of studies European cities3Tessum MW Anenberg SC Chafe ZA et al.Sources of ambient PM2·5 exposure in 96 global cities.Atmos Environ. 2022; 286119234Crossref Scopus (4) Google Scholar or have focused on the benefits of reductions in European emissions without specifying the city in which those benefits occur.8Gu Y Henze DK Nawaz MO Cao H Wagner UJ Sources of PM2·5-associated health risks in Europe and corresponding emission-induced changes during 2005–2015.GeoHealth. 2023; 7e2022GH000767Crossref Scopus (0) Google Scholar Khomenko and colleagues' work thus presents a novel quantification of how different sources contribute to health burdens from urban air pollution in Europe. Khomenko and colleagues6Khomenko S Pisoni E Thunis P et al.Spatial and sector-specific contributions of emissions to ambient air pollution and mortality in European cities: a health impact assessment.Lancet Public Health. 2023; 8: e546-e558Google Scholar identified a frequent and significant contribution of emissions from outside a city on its own health burden (ie, mean contribution of 86·5% for PM2·5 and 65·6% for NO2). Although perhaps surprising at first, these results are rationalised when considering the following points. First, pollution from PM2·5 and NO2 is an increasingly municipally controlled problem as the size of the city increases. The sheer number of smaller cities included in this study skews the mean urban contribution in a non-population-weighted manner. Second, as shown in the Article appendix, the estimates of city contributions to their own PM2·5 burden are actually substantially larger than previously reported values for nearly all European cities studied by Tessum and colleagues.3Tessum MW Anenberg SC Chafe ZA et al.Sources of ambient PM2·5 exposure in 96 global cities.Atmos Environ. 2022; 286119234Crossref Scopus (4) Google Scholar Lastly, emissions from a city not largely affected by its own emissions (eg, Nancy, France, where the local urban contribution is less than a fifth) might still contribute to air pollution elsewhere. For example, Gu and colleagues8Gu Y Henze DK Nawaz MO Cao H Wagner UJ Sources of PM2·5-associated health risks in Europe and corresponding emission-induced changes during 2005–2015.GeoHealth. 2023; 7e2022GH000767Crossref Scopus (0) Google Scholar quantified the effect of emissions in Europe on premature deaths from long-term PM2·5 exposure anywhere within Europe. In their research, they show that emissions from many cities in northeast Spain, central UK, northeast France, Luxembourg, east Germany, and Austria cause more premature deaths related to air pollution throughout Europe than within cities in these regions themselves. Emissions from Nancy, France, for example, cause more than 2-times more premature deaths from long-term PM2·5 exposure in Europe than in Nancy itself. Thus, the merits of urban-scale emission control policies should not be viewed solely through the lens of the benefits to the local urban population, the broader regional benefits should also be recognised. Khomenko and colleagues also emphasised the roles of residential and agriculture emissions (instead of factories, powerplants, vehicles, etc) for PM2·5-related mortality.6Khomenko S Pisoni E Thunis P et al.Spatial and sector-specific contributions of emissions to ambient air pollution and mortality in European cities: a health impact assessment.Lancet Public Health. 2023; 8: e546-e558Google Scholar The authors are not alone in calling out these sectors as the dominant contributors to the PM2·5 health burden in Europe.8Gu Y Henze DK Nawaz MO Cao H Wagner UJ Sources of PM2·5-associated health risks in Europe and corresponding emission-induced changes during 2005–2015.GeoHealth. 2023; 7e2022GH000767Crossref Scopus (0) Google Scholar, 9Lelieveld J Evansm JS Fnais M Giannadaki D Pozzer A The contribution of outdoor air pollution sources to premature mortality on a global scale.Nature. 2015; 525: 367-371Crossref PubMed Scopus (3451) Google Scholar Furthermore, the computational methods used for quantifying source contributions in such studies are important to note when considering how results are applied to pollutants that respond non-linearly to changes in emissions.10Clappier A Belis CA Pernigotti D Thunis P Source apportionment and sensitivity analysis: two methodologies with two different purposes.Geosci Model Dev. 2017; 10: 4245-4256Crossref Scopus (68) Google Scholar In particular, sensitivity-based studies, such as those by Khomenko and colleagues6Khomenko S Pisoni E Thunis P et al.Spatial and sector-specific contributions of emissions to ambient air pollution and mortality in European cities: a health impact assessment.Lancet Public Health. 2023; 8: e546-e558Google Scholar and Gu and colleagues,8Gu Y Henze DK Nawaz MO Cao H Wagner UJ Sources of PM2·5-associated health risks in Europe and corresponding emission-induced changes during 2005–2015.GeoHealth. 2023; 7e2022GH000767Crossref Scopus (0) Google Scholar can emphasise the role of the agriculture sector (because PM2·5 response to NH3 emissions is often non-linear). This result can occur because sensitivity-based studies emphasise contributions from emissions for which small (eg, 10–20%) changes elicit the largest response in pollutant concentrations, which is not the same as emissions that elicit the largest response in pollutant concentrations should emissions be reduced by 100%. The results of this study and other similar works are valuable for emphasising how recent, current, and future changes to emissions affect air pollution health burdens, which is often a more useful and actionable perspective than explaining where 100% of the air pollution mass originates. Efforts to better characterise second order pollutant responses to emissions are needed, particularly to incorporate non-linear species (ie, O3) into future analysis. I declare conduct of externally funded research on air pollution health impacts through grants from the US National Aeronautics and Space Administration, the US Enivronmental Protection Agency, the US National Institutes of Health, the Wellcome Trust, and the International Council on Clean Transportation. Spatial and sector-specific contributions of emissions to ambient air pollution and mortality in European cities: a health impact assessmentWe estimated source-specific air pollution health effects at the city level. Our results show strong variability, emphasising the need for local policies and coordinated actions that consider city-level specificities in source contributions. Full-Text PDF Open Access